The present invention relates to a charged particle beam irradiation system as part of a particle beam treatment system and an operating method for the charged particle beam irradiation system. More particularly, the invention relates to a charged particle beam irradiation system adapted to a particle beam treatment system for irradiating a cancer tumor with a charged particle beam (ion beam) of protons or carbon ions for treatment, and an operating method for the charged particle beam irradiation system.
In recent years, the rate of cancer incidence has been rising. With this trend has come progress in various methods for cancer treatment. Particle beam treatment is one method of radiation treatment involving irradiation of the cancer tumor with an accelerated charged beam particle such as a proton beam or a carbon ion beam for the destruction of cancer cell DNA. This method has been attracting attention in recent years because it is a minimally invasive technique that is less stressful on the body and permits higher quality of life after treatment.
The particle beam treatment system is made up of a charged particle beam generator, a high-energy beam transport system, an irradiation device, and a control device for controlling these components.
What is drawing attention in connection with the particle beam treatment system is the scanning irradiation method which, when an irradiation device irradiates a tumor with a charged particle beam in keeping with the tumor shape, involves causing the tumor to be irradiated with the beam through scanning by use of magnets.
According to the scanning irradiation method, the target tumor is divided into scanning regions called layers in the depth direction under the body surface. The plane of each layer is further divided into dose-controlled regions called spots. In this state, each layer plane is scanned with the charged particle beam for irradiation with the dose suitably controlled for each spot. A change from one irradiation plane to another is accomplished by varying the energy of the charged particle beam for irradiation.
Specifically, the value of the current for the scanning magnet is set. When a target spot is reached, that spot is irradiated with a set dose. When the irradiation dose has reached the set value, the next spot is reached. When the movement to the next irradiation position is completed, the charged particle beam is again extracted. Extraction and deactivation of the charged particle beam are repeated until the irradiation within one layer is finished. When the irradiation of one layer is completed, the beam energy is changed for the next layer, and the irradiation is repeated likewise. This is the so-called spot scanning method in which the charged particle beam for irradiation is turned off during movement from one irradiation spot to another.
The spot scanning method permits irradiation with the charged particle beam in keeping with the tumor shape and eliminates the need for patient-specific instruments such as the bolus and collimator required traditionally for the scatterer irradiation method. This makes it possible efficiently to irradiate the tumor with the charged particle beam supplied from the charged particle beam generator to the irradiation device.
Synchrotrons are used extensively as the charged particle beam generator for the particle beam treatment system. The radio frequency beam extraction method is one known method for extracting the beam from the synchrotron to the irradiation device.
The radio frequency beam extraction method involves increasing the amplitude of betatron vibrations of an orbiting charged particle beam by application of an extraction radio frequency voltage for extraction starting with particles having amplitudes exceeding a stability limit condition.
The extraction method above allows the set value of the magnet making up the synchrotron to be set constant during extraction. For this reason, the radio frequency beam extraction method offers enhanced orbiting stability of the extracted charged particle beam and provides highly accurate control of the position irradiated therewith. Also, because the extraction radio frequency voltage is turned off at the end of the irradiation of each spot, it is easy to stop the charged particle beam (e.g., see Nuclear Instruments and Method in Physics Research, A 489 (2002), pp. 59-67).